Reassessment of Sedimentary Evidence for Deep Contour Currents West of Iberia

Abstract

The 6.5 kHz GLORIA long range sidescan sonar images from west of Portugal were previously interpreted (Fig. 1) as showing evidence for mud waves and for drifts extending northwards from basement highs (Roberts and Kidd 1984; Gardner and Kidd 1987).A deep, northwards flowing contour current, existing over a long period at depths of between about 2,000 m and 4,000 m, was invoked to explain these observations. No profiles across these supposed contourite mud waves were published, although a profile across supposed mud waves at about 3,500 m in the Gulf of Cadiz (further south) had been shown (Gardner and Kidd 1983). Even this example could be due to downslope processes, as shown by (2000) for waves on other slopes. Open image in new window Fig. 1. Trends of features mapped from early GLORIA sidescan sonar data (Roberts and Kidd 1984) compared with the area of the OKEAN survey. Later analysis of the same GLORIA data by Gardner and Kidd (1987) differentiated channel-associated features and edges of seamounts from the majority of lineations, which were attributed to contourite mud waves. The Dom Carlos valley trend and the edges of seamounts were also seen on the OKEANdata (Fig. 2) but none of the other lineat ions were present on either OKEAN or high resolution seismic profiles Open image in new window Fig. 2. Mosaic of OKEAN long range sidescan sonar data. High backscatter shown in lighter tones